13 February 2008

The Most Primitive Stem-Bat

Man, lots of news pertaining to the evolution of flight lately. I saw a presentation on this bit of news at SVP last year—nice to see that it's been published. (Or, uh, will be tomorrow—today it's online early.)

Bats are mysterious creatures—not just in the poetic sense, but in the phylogenetic sense. What are they related to? Where did they come from? For a long time they were considered primate relatives—in fact, Linnaeus included them (as Genus Vespertilio) in his original version of Ordo Primates. In particular, they were often thought to be related to colugos (Cynocephalus), also known as "flying lemurs". Colugos aren't really lemurs, which are true primates, but they are relatives of primates (as are tree shrews, Tupaiidae). Like bats (and several other groups of therian mammal), colugos are arboreal and have large skin membranes used for aerial locomotion. Several characters seemed to unite bats and colugos, or at least put them in the same neighborhood, and this was the status quo for a while.

Then the whole placental tree got shaken up by molecular studies. Soon it became clear that bats were not at all close to colugos or primates, but were instead part of a large radiation with the imaginative name of Pegasoferae (previously mentioned here). Pegasoferae also includes Perissodactyla (horses, rhinos, tapirs), Carnivora (dogs, bears, weasels, seals, cats, hyenas, mongooses, civets, etc.), and Manis (pangolins, a.k.a. "scaly anteaters"—very odd creatures which look like pine cones).

The upshot is that we lost our model (colugos) for what a flightless stem-bat might look like. Now we have nothing (unless some fossil out there has not been recognized as such), so it's hard to tell much about how flight evolved in bats.

Enter a new species from Wyoming. Onychonycteris finneyi is now the most primitive known stem-bat, edging out the previous titleholder, Icaronycteris index. It still has wings, but it also has more primitive characters than any other known stem-bat. As an example, look at bat claws. Modern bats may or may not have a claw on their first manual digit (thumb). I. index has this, as well as a claw on its second manual digit (hence the name—"index finger"). But O. finneyi has claws on all five of its manual digits (hence its name—"onychus" means "claw").

O. finneyi is not a bat ancestor, since it coexisted with more derived stem-bats like I. index. But it does provide some clues as to the ancestral state of flying bats ("apo-chiropterans"). Scans of its ear canals reveal that, unlike modern bats and other known stem-bats, it did not have sonar capabilities. So, in stem-bats, flight evolved before echolocation.

Analysis of its proportions suggests that, like some modern bats, it probably had a flight style where it did not flap continuously, but had a pattern of fluttering up and then gliding down, fluttering up and then gliding down, and so on. (Sort of like a big, leathery butterfly.)

Unlike most modern bats, but like I. index, O. finneyi has a long tail. This evolutionary pattern is also seen in pterosaurs and birds (avialans), where the ancestral trait is a long tail, but the most successful later lineages evolve shortened tails (with occasional reversals).

The work of integrating fossils into the new paradigm of placental evolution is still in its early stages. Hopefully the characters in O. finneyi can help elucidate this problem, just as they have helped to elucidate the origins of bat flight. This fossil is an important clue in The Mystery of the Bat.